It is well known in the art of vapor or steam generation to control the level of unvaporized liquid within the vapor generator to an optimum level. A variety of control systems have developed, with the most prevalent being termed the three element controller.
A three element controller derives its name from the use of three monitoring elements to provide the necessary control input: the current unvaporized liquid level within the vapor generator, the current mass flow rate of vapor exiting the vapor generator, and the current mass flow rate of liquid entering the vapor generator. The output of each of the these monitoring elements is combined, usually by first determining the difference between the vapor flow rate out and the liquid flow rate in, comparing the current liquid level with an optimum reference level, and using the results as the input to a proportional and integral controller for generating a feed liquid control signal for regulating the mass flow of liquid into the vapor generator.
The magnitude of the various control signals and the degree of response of the proportional and integral controller thereto are parameters which must essentially be determined for each individual vapor generator wherein such a control system is employed. Such a determination, although not difficult given an existing vapor generator to be controlled, is highly individualistic and is usually limited to the normal operating range and transients of the vapor generator load. Large or unexpected, non-routine excursions in vapor generator loading, pressure, or feed liquid temperature can result in instability or unacceptable excursions of the liquid level within the vapor generator.
In particular, a full load rejection by a pressurized water nuclear reactor generating station can cause unacceptable swings in the liquid level of the vapor generators associated therewith. This occurs due to a combination of factors, including the basic physics of the steam generator design and the inability of the standard three element control system to accommodate such a large transient.
During a total load rejection, the first action occuring is usually a shutoff of the turbine admission steam valve which causes an immediate increase in the pressure within the vapor generator. As will be described in more detail in a following section of this specification, the increase in pressure causes the collapse of vapor voids which are present below the liquid surface within the steam generator and results in an immediate and substantial decrease in the liquid level within the vapor generator. The standard three element controller, in response to this observed decrease in the apparent liquid level, increases feed liquid flow to compensate. Meanwhile, the decreased vapor flow to the power turbines has caused a corresponding decrease in the flow of extraction vapor or steam to the feed liquid preheaters, resulting in a substantial decrease in feed liquid temperature.
This excessively subcooled feed liquid enters the vapor generator, causing a still further collapse of the vapor voids within the liquid inventory of the vapor generator, and further depresses the measured liquid level. Eventually the liquid inventory within the steam generator becomes heated again to a saturation point and the vapor voids are again present within the liquid inventory, but it is now too late for the three element control system to take any action to prevent the liquid level within the vapor generator from rising to an unacceptably and unworkably high level. The overfilling of the vapor generator with subcooled liquid at a time when the liquid level is greatly depressed will inevitably result in the level of liquid within the vapor generator rising uncontrollably above the optimum liquid level as the subcooled entering liquid is warmed to saturation.
This situation, common in vapor generators having a large inventory of unvaporized liquid, is only one of a number of areas in which the standard three element level controller is in need of improvement. A nonexhaustive list of such areas would include the response to the loss of all feed liquid heaters and the loss of a feed liquid pump.